1. Field of the Invention
The present invention relates generally to ADCs (analog to digital converter), and more specifically to techniques for improving signal-to-noise ratio (SNR) when using fewer bits than the number of output bits of an ADC as digital representation of a strength of an input signal.
2. Related Art
Analog to digital converters (ADCs) are used to generate a sequence of digital codes representing the strength of an input signal at corresponding time instances, and may be implemented in various forms such as successive approximation (SAR) ADC, pipelined ADC, etc., as is well known in the relevant arts.
Signal-to-Noise Ratio (SNR) is generally a measure of the accuracy with which the digital codes represent the strength of the corresponding sample of the input signal, and is typically measured as a ratio of the strength of the input signal as represented by a corresponding digital code to the strength of noise components (due to various error sources) represented by the digital code. It is generally desirable that the SNR of a ADC be high such that the digital values accurately represent the input analog signal.
One source of noise in the output of an ADC is quantization noise, which results from mapping an input signal strength to one of finite digital codes, and is proportional to the value of the (signal strength represented by) least significant bit (LSB) of the output digital code. SNR is generally given by the relation (6.02*H+1.76) decibels (dB), where H is the number of output bits in the digital code (assuming a linear relationship between the output codes and the input signal). Each digital code generated by an ADC may contain multiple output bits. In general, using more number of output bits (H) for a digital code results in a larger (better) value of SNR.
However, there may exist scenarios where it may be desirable to represent each digital code using a number of bits fewer than the number of output bits (H, in the description above) of an ADC. For example, when the digital codes generated by an ADC need to be transmitted, it may be desirable to use a fewer number of bits for the digital codes due to bandwidth limitations of the path on which the codes need to be transmitted. Accordingly, each digital code may be represented by a fewer number of bits to fit within the bandwidth constraints.
As another example, there are often government restrictions, which prohibit vendors from selling ‘ADCs’ providing digital codes with more bits than a pre-specified number (e.g., 11 bits). However, at least for economies of scale, a vendor may use internal ADCs operating with higher number of output bits (e.g., 15), but externally limit the number of bits in each digital code to the pre-specified number (11).
Using a fewer number of bits, however, results in a reduced SNR, at least for reasons noted above. Therefore, it may be desirable to improve the SNR when using fewer bits than the number of output bits of an analog to digital converter.
In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.